1. Field of the Invention
The present invention relates to zoom lenses and image pickup apparatuses having the zoom lenses. The zoom lenses are, for example, suitable for use in broadcast television cameras, video cameras, digital still cameras, or silver-halide film cameras.
2. Description of the Related Art
In recent years, there has been a demand for large diameter, wide field angle, high zoom ratio, and high optical performance zoom lenses for use in image pickup apparatuses, such as television cameras, silver-halide film cameras, digital cameras, and video cameras.
An example of large diameter, wide field angle, and high-zoom-ratio zoom lens is a positive-lead four-unit zoom lens, which consists of four lens units with the lens unit closest to the object side having positive refractive power.
A four-unit zoom lens of this type consists of, in order from the object side to the image side, a first lens unit that includes a focusing lens unit and has positive refractive power; a second lens unit that performs zooming and has negative refractive power; a third lens unit that corrects image plane variation caused by zooming; and a fourth lens unit that forms an image and has positive refractive power.
U.S. Pat. Nos. 5,745,300, 6,545,818, and 6,124,982 disclose four-unit zoom lenses using a three-unit inner-focus system and having a zoom ratio in the range from about 8× to 17.6×, a photographing field angle (field angle) at the wide-angle end in the range from about 65° to 95°, and a minimum shooting distance from the lens surface of about 0.3 m.
Herein, the three-unit inner-focus system refers to a focusing system in which the first lens unit having positive refractive power consists of, in order from the object side to the image side, a first lens subunit having negative refractive power, a second lens subunit having positive refractive power, and a third lens subunit having positive refractive power, and in which the second lens subunit is moved toward the image plane when the zoom lens shifts focus from an object at infinity to an object at a very short distance.
U.S. Pat. No. 5,745,300 discloses a four-unit zoom lens in which the first lens subunit has an aspherical surface. This zoom lens effectively corrects variation of the spherical aberration that occurs when the zoom lens is focused on an object at a very short distance while being set at the telephoto end, and negative distortion or curvature of field that occurs when the zoom lens is set at the wide-angle end.
U.S. Pat. No. 6,545,818 discloses a four-unit zoom lens in which the second lens subunit and the third lens subunit each have an aspherical surface. This zoom lens effectively corrects the negative distortion that occurs when the zoom lens is set at the wide-angle end, and the positive distortion that occurs when the zoom lens is set near a zooming position of zoom ratio Z1/4.
U.S. Pat. No. 6,124,982 discloses a four-unit zoom lens in which the first lens subunit has an aspherical surface. This zoom lens effectively corrects the negative distortion that occurs when the zoom lens is set at the wide-angle end.
It is relatively easy to increase the diameter ratio and the zoom ratio of a positive-lead four-unit zoom lens. However, as the zoom ratio increases, variation of aberrations also increases. This makes it difficult to provide a zoom lens having high optical performance over the entire zoom range and object distance range.
The provision of an aspherical surface in the first lens unit of a four-unit zoom lens makes it easy to correct aberrations and produce a zoom lens having high optical performance over the entire object distance range.
However, even if the first lens unit has an aspherical surface, unless the position and shape of the aspherical surface as well as the configuration of the first lens unit having the aspherical surface are appropriately set, effective correction of aberrations to produce a zoom lens having high optical performance over the entire object distance range is difficult.
When a four-unit zoom lens uses a three-unit inner-focus system, appropriate setting of the configuration of the second lens subunit is an important factor to reduce variation of aberrations caused by focusing and increase the field angle and the zoom ratio.